In a single antenna wireless communication, a single transmitter communicates with a single receiver over a wireless channel. In a multiple antenna wireless communication, multiple transmitters communicate with one or more receivers over a wireless channel. Each receiver can determine multiple outputs corresponding to the multiple transmitters. This is commonly called Multiple-In, Multiple-Out (MIMO), and is a relatively recent addition to the wireless field.
Ideally, any transmitted signal appears at the receiver as a received signal, where the transmitted and received signals are identical except for a time delay caused by the time the transmitted signal takes to traverse the channel. However, many wireless channels cause fluctuations in the received signal. For instance, a receiver may be moving away from the transmitter, which will cause a temporal fluctuation. As another example, the transmitted signal may bounce off of multiple buildings on the way to the transmitter. These different and slightly delayed signals can cause destructive or constructive interference. This type of interference can cause spectral fluctuations.
There are techniques for compensating for these fluctuations. For instance, many techniques send “pilots,” which are known symbols, during a communication. A receiver uses the pilots to attempt to correct for either temporal or spectral fluctuations, but not both. However, pilot usage can get very high for the MIMO communication systems. Each pilot takes the space of data, so more pilots means less bandwidth for data transmission. Moreover, each of these techniques to correct temporal or spectral fluctuations of wireless channels causes its own set of problems. A need therefore exists for techniques that correct for temporal and spectral fluctuations and also allow for MIMO communication systems.